1. Field of the Invention
This invention relates to a resonator adapted to vibrate in a thickness-shear mode, of the kind comprising a plate of piezo-electric material and plate vibrating means that include a first electrode on a first surface of the plate, a second electrode on the second surface of the plate opposite the first electrode, means for fastening the resonator mechanically to a support and means for connecting the electrodes electrically to an oscillator circuit including terminals on said plate and conductors connected to said terminals and to said electrodes.
A resonator of this type, of which several forms of embodiment are described in e.g. U.S. Pat. No. 4,071,797, is usually associated with an appropriate electronic circuit to form an oscillator which provides an alternating signal having a very stable frequency. Oscillator circuits of this type are used, e.g., in electronic timepieces for which they act as a time base.
The vibration of a resonator of this type is generated by the alternating electric field created between its excitation electrodes by a voltage, also alternating, applied to the electrodes by the electronic circuit to which they are connected.
The mechanical energy necessary for maintaining this vibration is of course delivered, in the form of electric energy, by the electric supply source of the circuit.
If this mechanical energy were confined to the zone of the resonator located between the excitation electrodes, the quantity of electric energy that the supply source would have to deliver to maintain the vibration would be minimal.
However, part of the mechanical energy generated in the zone located between the electrodes leaves this zone. The remainder of the resonator therefore also vibrates, with an amplitude which decreases exponentially away from this zone. The mechanical energy associated with this vibration decreases of course in the same manner.
If the dimensions of this resonator are large enough for the amplitude of this vibration to be virtually nil in the zone where the resonator is fastened to its support due to its exponential decrease, the mechanical energy associated with this vibration is also virtually nil in the latter zone.
If, on the contrary, the dimensions of the resonator are relatively small, the amplitude of this vibration and the mechanical energy associated with it are not nil in the zone where the resonator is fastened. This mechanical energy is dissipated wastefully in the fastening, thus causing a decrease in the quality factor of the resonator.
2. Prior Art
The current trend being to make resonators of the smallest possible size, many proposals have been made for decreasing the amplitude of the vibration of the plate in the zone where it is fastened and thus the quantity of energy dissipated wastefully into the fastening, in order to produce resonators having a good quality factor in spite of their small size.
U.S. Pat. No. 4,124,809, for example, suggests making the ends of the resonator thinner outside the zone where the excitation electrodes are located.
The actual production of such a resonator however, is complicated in practice, and it cannot be mass-produced at low cost.